Container and method of mitigating metal-contact contamination of polysilicon

ABSTRACT

The present disclosure concerns reduction or mitigation of metal-contamination of polycrystalline silicon when held or stored in containers at least partially constructed of metal and/or having polysilicon contact surfaces at least partially of metal. In particular, the disclosure relates to a method of mitigating metal contamination of polycrystalline silicon from contact with a metal surface of a container by providing the surface with a protective layer comprising a microcellular elastomeric polyurethane.

CROSS REFERENCE TO RELATED APPLICATION

This claims the benefit of U.S. Provisional Application No. 61/724,844,filed Nov. 9, 2012, which is incorporated in its entirety herein byreference.

FIELD

The present disclosure relates to a modified container for polysiliconand a method of mitigating metal contamination of polysilicon containedwithin.

BACKGROUND

Silicon of ultra-high purity is used extensively for applications in theelectronic industry and the photovoltaic industry. The purity demandedby industry for these applications is extremely high and frequentlymaterials with only trace amounts of contamination measured at the partper billion levels are deemed acceptable. By rigorous control of thepurity of the reactants used to manufacture polycrystalline silicon itis possible to produce such high purity polycrystalline silicon but thenextreme care must be taken in any handling, packaging or transportationoperations to avoid post contamination. At any time the polycrystallinesilicon is in contact with a surface there is a risk of contamination ofthe polycrystalline silicon with that surface material. If the extent ofcontamination exceeds certain industrial stipulations then the abilityto sell the material into these end applications may be restricted oreven denied. In this respect minimizing contact metal contamination is aprimary concern if silicon performance criteria in the semiconductorindustries are to be attained.

While addressing the matter of avoiding metal contact contaminationthere is a need to provide a container which has an enhanced servicelife and can avoid the need to frequently change out the containerand/or replace the resin member.

SUMMARY

According to one aspect, this disclosure concerns a method of reducingor eliminating contamination of granular polysilicon, during storage ortransportation, from contact with an inner metal surface of a container,the method comprises placing polysilicon in a container provided with aprotective liner comprising microcellular elastomeric polyurethane on atleast a portion of an inner metal surface of the container, the linerbeing located such that the polysilicon is shielded from contact withthe inner metal surface.

According to a further aspect, this disclosure relates to a containersuitable for holding granular polysilicon. The container is a containerat least partially constructed of metal and having an inner metalsurface that defines a region to contain polysilicon, and portions ofthe inner surface that have contact with the polysilicon are providedwith a protective liner comprising microcellular elastomericpolyurethane, thereby reducing or eliminating metal contamination of thepolysilicon.

According to yet a further aspect, this disclosure relates to an articlecomprising:

-   -   a) a container suitable for holding granular polysilicon,        wherein the container has an inner metal surface defining a        region for holding polysilicon, and portions of the inner metal        surface are provided with a protective liner comprising        microcellular elastomeric polyurethane; and    -   b) granular polysilicon located within the container, wherein        the polysilicon is in contact with the protective liner, thereby        reducing or eliminating metal contamination of the polysilicon.

The foregoing and other objects, features, and advantages will becomemore apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a lined container suitablefor holding polysilicon.

FIG. 2 is a schematic perspective view of a lined hopper and dischargechute suitable for holding and discharging polysilicon.

DETAILED DESCRIPTION

Unless otherwise stated, all numbers and ranges presented in thisapplication are Approximate—within the scientific uncertainty values forthe tests required to determine such number values and ranges, as knownto those of ordinary skill in the art.

The expression “storage container” in the context of this disclosure mayinclude a bin, a drum, a chute, a hopper or a crushing bed at leastpartially of metal construction and/or having polysilicon contactsurfaces at least partially of metal. The container may be associatedwith the processing and manufacturing operation of polysilicon and/orthe packaging and transportation of polysilicon. The container may be anarticle of commerce when polysilicon is held within the container.

The polysilicon held within the container is not limited to anyparticular source and may include particulate or granular silicon usedas seed in a fluid bed reactor method of producing polysilicon, or theproduct harvested from such a process. The polysilicon contained withinthe container can also be that obtained from a Siemens process and maybe in the form silicon dendrites obtained according to the methoddisclosed in the U.S. patent publication U.S.2003/0150378.

In one embodiment, the polysilicon storage container is a containerintended for use with polysilicon associated a fluid bed manufacturingprocesses for crystalline polysilicon, typically referred to asgranulate or granular silicon. Granulate silicon encompasses materialhaving an average size in its largest dimension of from about 0.01micron to as large as 15 millimeters. More typically, the majority ofgranulate silicon will have an average particle size of from about 0.1to about 5 millimeters and be essentially spheroid in form and devoid ofthe presence of any sharp or acute edge structure.

Metal contamination of silicon held within the container is caused bydirect contact of the silicon with a metal surface, or by entrainment oferosion or wear products as discrete metal particles within the bulk ofthe silicon mass (e.g., particulate polysilicon). In disclosedembodiments, the contamination is reduced or avoided by the presence ofa protective liner at least partially covering one or more inner metalsurfaces of a container and wherein the protective liner comprisesmicrocellular elastomeric polyurethane. In the context of thisapplication the term “polyurethane” may also include materials where thepolymer backbone comprises polyureaurethanes orpolyurethane-isocyanurate linkage. In some embodiments, at least 50% orat least 75% of the surface is covered by a protective liner asdisclosed herein. In certain embodiments, the surface is completelycovered by the protective liner. “Completely” should be taken asessentially free from defects from a practical point of view. Whenpolysilicon is added to the container, the polysilicon contacts theprotective liner instead of a bare metal surface.

FIG. 1 is a cross-sectional schematic diagram of an exemplarypolysilicon storage container 10 including a container 20 at leastpartially of metal construction and a protective liner 30 at leastpartially covering one of more inner surfaces 22 of the container 20.Particulate or granular polysilicon 40 located within the container 20is in contact with the protective liner 30.

FIG. 2 is a schematic perspective view of an exemplary polysiliconcontainer system 100 including a storage hopper 110 and a dischargechute 120. One or both of storage hopper 110 and chute 120 is at leastpartially constructed of metal. A protective liner 130 at leastpartially covers one or more inner metal surfaces 112 of hopper 110,and/or at least partially covers one or more inner metal surfaces 122 ofchute 120. Particulate or granular polysilicon 140, located withinhopper 110 and chute 120, is in contact with the protective liner 130.

The term “elastomeric” refers to a polymer with elastic properties,e.g., similar to vulcanized natural rubber. Thus, elastomeric polymerscan be stretched, but retract to approximately their original length andgeometry when released.

The term “microcellular” generally refers to a foam structure havingpore sizes ranging from 1-100 μm. Microcellular materials typicallyappear solid on casual appearance with no discernible reticulatestructure unless viewed under a high-powered microscope. With respect toelastomeric polyurethanes, the term “microcellular” typically is definedby density, such as an elastomeric polyurethane having a bulk densitygreater than 600 kg/m³. Polyurethane of lower bulk density typicallystarts to acquire a reticulate form and is generally less suited for useas the protective coating described herein.

Microcellular elastomeric polyurethane suitable for use in the disclosedapplication is that having a bulk density of 1150 kg/m³ or less, and aShore Hardness of at least 65A. In one embodiment the elastomericpolyurethane has a Shore Hardness of up to 90A, such as up to 85A; andfrom at least 70A. Thus, the Shore Hardness may range from 65A to 90A,such as 70A to 85A. Additionally, the suitable elastomeric polyurethanewill have a bulk density of from at least 600 kg/m³, such as from atleast 700 kg/m³ and more preferably from at least 800 kg/m³; and up to1150 kg/m³, such as up to 1100 kg/m³ or up to 1050 kg/m³. Hence, thebulk density may range from 600-1150 kg/m³, such as 800-1150 kg/m³, or800-1100 kg/m³. The bulk density of solid polyurethane is understood tobe in the range of 1200-1250 kg/m³. In one embodiment, the elastomericpolyurethane has a Shore Hardness of from 65A to 90A and a bulk densityof from 800 to 1100 kg/m³.

Elastomeric polyurethane can be either a thermoset or a thermoplasticpolymer; this presently disclosed application is better suited to theuse of thermoset polyurethane. Microcellular elastomeric polyurethanehaving the above physical attributes is observed to be particularlyrobust, and withstands the abrasive environment and exposure toparticulate granulate silicon eminently better than many othermaterials.

The protective liner or coating on the inner metal surface of thecontainer typically will be present in an overall thickness of from atleast 0.1, such as from at least 0.5, from at least 1.0, or from atleast 3.0 millimeters; and up to a thickness of about 10, such as up toabout 7, or up to about 6 millimeters. Thus, embodiments of thedisclosed protective liner may have a thickness from 0.1-10 mm, such as0.5-7 mm or 3-6 mm. The protective liner can be a plastic laminatestructure comprising an outer polyurethane lamella being in contact withthe silicon. However, in one embodiment, it is contemplated that themajority or the entirety of the protective liner be the elastomericmicrocellular polyurethane.

The placement of the protective liner within the 1 container may beachieved by obtaining the elastomeric microcellular polyurethane in theform of a sheet; cutting the sheet to form one or more parts shaped toconform to the interior size and geometry of the cavity of thecontainer; coating at least a portion of the inner metal surface of thecontainer with an adhesive; and then bringing into contact with theadhesive the polyurethane sheet thereby forming a laminate structure ofmetal/adhesive/polyurethane. In another embodiment, the adhesive isapplied to an outer surface of the polyurethane material (i.e., thesurface facing the inner metal surface of the container), and thepolyurethane/adhesive is brought into contact with the inner metalsurface of the container. In yet another embodiment, the adhesive isapplied to both the inner metal surface and the surface of thepolyurethane material. Desirably, the adhesive is selected to becompatible with the metal and polyurethane, thereby providing adequateadhesion and avoiding subsequent delamination and/or failure. Suitableadhesives include, but are not limited to, a polyurethane, isocyanate orepoxy-based adhesive. Optionally the metal surface and/or the backsurface of the polyurethane sheet to be brought into the contact withthe adhesive may be roughened and/or treated with a primer substance toenhance the integrity and strength of the adhesive bond.

In another embodiment, such as in the instance of complex inner geometryand shape of the container, the placement of the microcellularpolyurethane protective liner can be achieved by a spray-coatingtechnique. In this technique, the precursor materials to thepolyurethane are brought together and sprayed directly as a reactivemass onto the exposed metal surface, which on reacting out and curingprovides the desired protective liner. Preparing the protective liner inthis manner offers the advantage of good adhesion of the polyurethane tothe metal surface while avoiding seams inherent to the above-mentionedplacement procedure involving cut sheets. An alternative applicationprocedure to spray coating is in situ casting of the polyurethane, whichhas the additional advantage of providing an article with substantiallysmooth continuous polyurethane surfaces.

Procedures for the manufacture of microcellular polyurethane elastomersare well known to a person skilled in the in the art and in generalcomprises reacting a polyisocyanate with a polyether polyol giving apolyether polyol-based polyurethane (“Pe-PU”), or alternatively byreaction of a polyisocyanate with a polyester polyol giving a polyesterpolyol-based polyurethane (“Pst-PU”). Polyester polyol-basedpolyurethane elastomers are typically observed as having physicalproperties better suited to the presently disclosed application comparedto the polyether polyol-based polyurethane elastomer and hence are thepreferred elastomeric polyurethane for use herein. Exemplarypublications teaching the preparation of microcellular polyurethaneelastomers include: U.S. Pat. No. 4,647,596; U.S. Pat. No. 5,968,993;U.S. Pat. No. 5,231,159; U.S. Pat. No. 6,579,952; U.S.2002/111,453 andU.S.2011/003103. Procedures for the manufacture of polyurethane-linedmetal articles are also known to a person skilled in the art andexemplified by publications including U.S.2005/189,028; GB 2,030,669;U.S. Pat. No. 5,330,238; or JP52-20452. Procedures for manufacture ofpolyurethane or polyurethane-urea and articles therewith by spraytechniques are also known to a person skilled in the art and exemplifiedby publications including U.S.2008/305,266; WO2012/005351; U.S. Pat. No.6,399,736; U.S. Pat. No. 6,747,117; U.S. Pat. No. 7,736,745; and U.S.Pat. No. 6,730,353.

It is observed that such polyurethane-lined containers are able tosatisfactorily mitigate metal contamination of the granular polysiliconduring its storage and/or transportation. Abrasive failure or fracturesof the polyurethane lining during the transportation of granulatepolysilicon is surprisingly low and absent. Organic or carboncontamination of the polysilicon is also observed to be minimal and notdistracting from the overall quality of the polysilicon.

The specific examples included herein are for illustrative purposes onlyand are not to be considered as limiting to this disclosure.

EXAMPLE 1

The advantageous selection of microcellular elastomeric polyurethane asa protective liner for the container relative to other potential linermaterials is illustrated by studying the abrasion resistance propertiesof the potential liner materials. Abrasion is considered the mostlylikely means of failure of the protective liner. Hence an acceleratedabrasion wear test has been deployed to identify the material of choice.

Accelerated abrasion wear testing of a variety of plastic resinsconsidered as potential candidates for deployment as the protectivecoating layer in the presently disclosed application has been conducted.The test procedure has been designed to mimic the demanding conditionsthat might occur in a typical operation associated with the manufacture,transfer and storage of granulate polysilicon. The general procedureconsists of subjecting coupons (3″×3″×0.5″ (7.6×7.6×1.3 cm)) of plasticresins to abrasive impact erosion by particulate polysilicon andobserving the change to the surface of the coupon after a given time.The particulate or granular polysilicon used consists of essentiallysmooth spheroid particles having an average (95%) particle size of from0.9-1.2 mm. The polysilicon particles are caused to impact the largesurface of the plastic coupons, at a focused central point, by beingcarried in a jetted air stream operating at a pressure of about 15 psi(103,420 Pa) and estimated as conferring a particle velocity of from 45to 55 feet/sec (13.7-16.8 m/s). The orientation of the jetted air streamwas set to provide a fixed given impact angle, relative to the couponsurface. This configuration exposes the coupon surface to passage ofapproximately 24 kg/hour of granular polysilicon material. The wear andabrasive loss on the coupon being observed by formation of a surfacecrater the depth of which is measured after a set continuous exposuretime to polysilicon.

Table 1 below presents the observations; it is clearly seen thatelastomeric polyurethanes have superior performance as evidenced by thereduced crater formation.

Comparative Comparative Example 1 Example 2 Example 1 Example 2 CouponPolypropylene Ethylene- Polyurethane Polyurethane Materialtetrafluoroethylene Elastomer Elastomer (Polyether (Polyesterpolyol-based) polyol-based) Density 900 1700 1100 1100 (kg/m³) Shore 67D 67 D 80 A 74 A Hardness Exposure 1500 1500 1500 1500 1500 1500 15001500 Time (mins) Impact 15 30 15 30 15 30 15 30 Angle (Degrees) Crater0.46 cm Exceeded Not 1.0 cm 0.10 cm 0.13 cm <0.025 cm 0.025 cm Depth(cm) 1.3 cm Observed

While the foregoing discussion focuses on embodiments of a linedcontainer for storage and/or transport of polysilicon, it will beunderstood by a person of ordinary skill in the art that there are othermaterials of importance in the semiconductor industry, such asgermanium, where similar management to avoid foreign metal contactcontamination is also highly desirable.

In view of the many possible embodiments to which the principles of thedisclosure may be applied, it should be recognized that the illustratedembodiments are only preferred examples and should not be taken aslimiting the scope of the disclosure. Rather, the scope of thedisclosure is defined by the following claims.

We claim:
 1. A method of reducing or eliminating contamination ofgranular polysilicon, during storage or transportation, from contactwith an inner metal surface of a container, the method comprisingplacing granular polysilicon in a container provided with a protectiveliner comprising microcellular elastomeric polyurethane on at least aportion of an inner metal surface of the container, the liner beinglocated such that the granular polysilicon is shielded from contact withthe inner metal surface.
 2. The method of claim 1 wherein thepolyurethane has a Shore Hardness of from 65A to 90A and a bulk densityof from 800 to 1150 kg/m³.
 3. The method of claim 1 wherein theprotective liner has an overall thickness of from 0.1 to 10 millimeters.4. The method of claim 1 wherein the granular polysilicon is essentiallyspheroid in form with an average size in its largest dimension of from0.01 micron to 15 millimeters and is devoid of sharp or acute edgestructure.
 5. A container suitable for holding granular polysilicon, thecontainer comprising: a container comprising an inner metal surface thatdefines a region to contain granular polysilicon; and a protective linercomprising microcellular elastomeric polyurethane on at least a portionof the inner metal surface facing the region, wherein the granularpolysilicon is in contact with the protective liner when the granularpolysilicon is present in the region, and wherein the polyurethane has aShore Hardness of from 65A to 90A and a bulk density of from 800 to 1150kg/m³.
 6. The container of claim 5 wherein the protective liner has anoverall thickness of from 0.1 to 10 millimeters.
 7. The container ofclaim 5 wherein the granular polysilicon is essentially spheroid in formwith an average size in its largest dimension of from 0.01 micron to aslarge as 15 millimeters and is essentially devoid of sharp or acute edgestructure.
 8. A method of making the container of claim 5, the methodcomprising placing a protective liner comprising microcellularelastomeric polyurethane within a container comprising an inner metalsurface, thereby covering at least a portion of the inner metal surfacewith the protective liner.
 9. The method of claim 8, wherein placing theprotective liner within the container comprises (a) spray coatingpolyurethane onto the portion of the inner surface, or (b) forming theprotective liner by in situ casting.
 10. The method of claim 8, whereinplacing the protective liner within the container further comprises:obtaining a sheet of elastomeric microcellular polyurethane; cutting thesheet to form one or more elastomeric microcellular polyurethane partsshaped to conform to the interior size and geometry of the regiondefined by the inner metal surface of the container; coating the innermetal surface of the container with an adhesive; and bringing the one ormore elastomeric microcellular polyurethane parts into contact with theadhesive, thereby forming a laminate structure ofmetal/adhesive/polyurethane.
 11. The method of claim 8, wherein placingthe protective liner within the container further comprises: obtaining asheet of elastomeric microcellular polyurethane; cutting the sheet toform one or more elastomeric microcellular polyurethane parts shaped toconform to the interior size and geometry of the region defined by theinner metal surface of the container; coating an outer surface of theone or more polyurethane parts with an adhesive to form one or moreadhesive-coated polyurethane parts; and bringing the one or moreadhesive-coated polyurethane parts into contact with the inner metalsurface of the container, thereby forming a laminate structure ofmetal/adhesive/polyurethane.
 12. An article comprising: a) a containersuitable for holding polysilicon, wherein the container has an innermetal surface that defines a region to contain polysilicon, and at leasta portion of the inner surface is provided with a protective linercomprising microcellular elastomeric polyurethane; and b) granularpolysilicon located within the container, wherein the polysilicon isshielded from contact with the inner metal surface by the protectiveliner.
 13. The article of claim 12 wherein the granular polysilicon isessentially spheroid in form with an average size in its largestdimension of from 0.01 micron to as large as 15 millimeters and isessentially devoid of sharp or acute edge structure.